The present invention relates to the fabrication of semiconductor integrated circuits (IC""s). More particularly, the present invention relates to methods and apparatuses for the characterization of microelectronic feature quality on IC""s.
The fabrication of an integrated circuit requires the various materials comprising the device to be patterned into required circuit elements. This patterning operation is most often accomplished by the deposition of a uniform film layer of a desired material. A xe2x80x9cmaskxe2x80x9d is then formed on the film utilizing a photosensitive material, and finally the exposed material is etched away leaving the desired circuit elements.
The yield and performance of an integrated circuit can be critically dependent upon subtle feature characteristics of the etched circuit elements or features. Some of these important feature characteristics are: line width loss (undercut or xe2x80x9cbiasxe2x80x9d), sidewall angle (slope), surface roughness, the presence of residual material at step edges (xe2x80x9cfencesxe2x80x9d or xe2x80x9cstringersxe2x80x9d), and the contact angle at the base of the etched feature (xe2x80x9cfootxe2x80x9d). These etch characteristics result from the complex interaction of the etching chemistry, the plasma physics, and the etch system design and maintenance.
The function of semiconductor equipment manufacturers is to develop hardware, processes, and control systems capable of reliably and reproducibly creating the specific set of conditions required to produce the desired etched feature characteristics. The ability to develop and optimize the etch process and hardware is critically dependent upon the availability of sensors and instruments capable of measuring these important characteristics. What can not be measured, can not be optimized or reliably reproduced.
Furthermore, the time delay between when the etch takes place and when the etch characteristics are measured is of great importance. Measurements that are made while the etch is being performed (e.g., etch rate via endpoint detection) are generally used to provide immediate process feedback, and thus maintain optimal results. Measurements that are performed immediately after the etch process, are typically used for fault detection and to provide xe2x80x9crun-to-runxe2x80x9d compensation for process drift or hardware aging. Measurements which require significant time delays or human interaction and interpretation are generally used for basic process and hardware development but are not, in general, useful for optimizing and maintaining the process.
Many of the subtle but critical etch characteristics (e.g., sidewall angle, stringer formation, residue, etc.) can presently only be monitored through the use of Scanning Electron Microscopy (SEM) or other complex instrumentation. These techniques are time consuming, very localized, frequently damaging to the wafer, and require substantial human evaluation and interpretation. As such, SEM characterization is intensively used during process and hardware development, but is of limited use in maintaining a process at its optimum condition. SEM""s main application in integrated circuit manufacturing is in the detailed analysis and evaluation of problems detected via some other means, such as yield decreases. Since subtle changes in the etch characteristics, which typically result in yield loss, are generally not actively monitored, substantial numbers of wafers are placed at risk.
Another technique for analyzing wafer quality is Scatterometery. Scatterometery is based upon the analysis of light reflected or scattered from the surface of a wafer being evaluated. Scatterometery based sensors are presently available which can measure average surface roughness, estimate feature profile, and determine feature spacing, periodicity, and height. These sensors typically utilize monochromatic laser light reflected from specially designed periodic, diffraction grating like structures to monitor the characteristics of the features. The requirement for a special test structure on the wafer severely limits and complicates the use of these instruments for process control and/or real time fault detection. Moreover, different and specific configurations are required for the measurement of different feature attributes.
In view of the above, what is needed are methods and systems for providing an indication of wafer quality that are not time consuming, damaging to the wafer, or that require substantial human evaluation and interpretation. In addition, the methods should not require special test structures on the wafer, and should be economically viable.
The present invention addresses these needs by providing a system and method for characterizing the quality of microelectronic features utilizing broadband white light. In one embodiment, a highly collimated light source illuminates an area of a first wafer using multi-spectral light. Preferably, the highly collimated light source has an angular spread of less than xc2x11xc2x0, and more preferably, less than xc2x10.5xc2x0. The angular distribution of the light scattered from the first wafer is then measured. Generally, the angle of the light source, detector, or both is altered and an angular distribution measurement taken at each angle, producing a scatter signature for the first wafer. Finally, the scatter signature of the first wafer is compared with a known scatter signature of a second wafer of good quality to determine the quality of the first wafer.
In another embodiment, an apparatus for characterizing the quality of microelectronic features utilizing broadband white light is disclosed. The apparatus includes a broadband collimated light source, which is suitable for illuminating a surface of a first wafer with a light beam. The apparatus further includes a light detector, which is suitable for sensing light scattered from the illuminated surface of the first wafer. Finally, a computer for comparing a scatter signature of the first wafer with a known scatter signature of a second wafer of good quality is included.
In yet another embodiment of the present invention, a method for making an integrated circuit structure having monitored feature characteristics is disclosed. The method begins by illuminating an area of a first wafer using a highly collimated light source, wherein the light source produces broadband multi-spectral light. Preferably, the highly collimated light source has an angular spread of less than xc2x11xc2x0, and more preferably, less than xc2x10.5xc2x0. The angular distribution of the light scattered from the first wafer is then measured. Generally, the angle of the light source, detector, or both is altered and an angular distribution measurement taken at each angle, producing a scatter signature for the first wafer. The scatter signature of the first wafer is then compared with a known scatter signature of a second wafer of good quality to determine the quality of the first wafer. Finally, the wafer is processed through a series of semiconductor processes to form the integrated circuit.
Advantageously, the present invention provides an indication of the wafer quality in a timely manner. In addition, the present invention does not require substantial human evaluation and interpretation since the scatter signatures may be readily analyzed on a computer system. Finally, since the present invention provides wafer quality data without the need of complex equipment, the cost of the system remains relatively low.